THE HEART AND THE ARTERIES. 71 



their larger and smaller branches, which proceed from it 

 and ramify through the body, to terminate eventually in 

 the blaod sinuses, which represent the veins of the 

 higher animals. 



When the carapace is removed from the middle of the 

 region which lies behind the cervical groove, that is, 

 when the dorsal or tergal wall of the thorax is taken 

 away, a spacious chamber is laid open which is fuU of 

 blood. This is the cavity already mentioned as the peri- 

 cardium (fig. 15, p), though, as it diifers in some respects 

 from that which is so named in the higher animals, it will 

 be better to term it the pericardial sinus. 



The heart (fig. 15, h), lies in the midst of this sinus. It 

 is a thick muscular body (fig. 16), with an irregularly hexa- 

 gonal contour when viewed from above, one angle of the 

 hexagon being anterior and another posterior. The lateral 

 angles of the hexagon are connected by bands of fibrous tis- 

 sue {ac) with the walls of the pericardial sinus. Otherwise, 

 the heart is free, except in so far as it is kept in place by the 

 arteries which leave it and traverse the walls of the peri- 

 cardium. One of these arteries (figs. 5, 12, and 1 6, saa), 

 starting from the hinder part of the heart, of which it 

 is a sort of continuation, runs along the middle line of 

 the abdomen above the intestine, to which it gives ofi^ 

 many branches. A second large artery starts from a 

 dilatation, which is common to it with the foregoing, but 

 passing directly downwards (figs. 12 and 15, sa, and fig. 16, 

 St. a), either on the right or on the left side of the intestine, 



